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Up to now, oriented surface crystallization phenomena are discussed controversially, and related studies are restricted to few glasses. For silicate glasses we found a good correlation between the calculated surface energy of crystal faces and oriented surface nucleation. Surface energies were estimated assuming that crystal surfaces resemble minimum energy crack paths along the given crystal plane. This concept was successfully applied by Rouxel in calculating fracture surface energies of glasses. Several oriented nucleation phenomena can be herby explained assuming that high energy crystal surfaces tend to be wetted by the melt. This would minimize the total interfacial energy of the nucleus. Furthermore, we will discuss the evolution of the microstructure and its effect on the preferred crystal orientation.
Up to now, the mechanisms of surface nucleation and surface-induced texture formation are far from being understood. Corresponding phenomena are discussed hypothetically or even controversial, and related studies are restricted to very few glasses. As a main drawback, the vast majority of previous work on oriented growth of surface crystals do not consider the possible effects of glass surface treatments (like polishing defects, adhered particles and cracks, …) or ambient annealing conditions (vacuum or atmospheric pressure) which both can influence crystal nucleation and growth. Moreover, very few observations of crystal orientation related to surface crystallization were focused on separately growing crystals. In conclusion, up to now, no systematic studies on initially oriented crystal growth or nucleation from defined active nucleation sites have been pursued.
Therefore, the main objective of the proposed project is to gain a basic understanding of the mechanisms of surface-induced microstructure formation in glass ceramics. We shall answer the question whether preferred orientation of surface crystals is the result of oriented nucleation or caused by other orientation selection mechanisms acting during early crystal growth. In both cases, crystal orientation may be caused by the orientation of the glass surface itself or the anisotropy and orientation of active surface nucleation defects.
New techniques of experimental characterization of surface-crystallized glasses shall be applied to find answers to the questions posed above. Based on a controlled preparation and annealing of the glass sample surfaces, that includes the defined creation of active nucleation defects, and followed by the microstructural characterization especially during the very early growth stages of separated surface crystals, the orientation relations between defect and initiated surface crystal will be analyzed. In order to gain new insights, a couple of entirely new microstructure diagnostics techniques, including chemically contrasted XRM, laser-based sample preparation for XRM, and 3D electron backscatter diffraction shall be developed or applied.
This way, using X-ray microscopy as a pivotal technique for the non-destructive 3D characterization of relatively large samples at superior spatial resolution (but also complemented by other microstructure diagnostics techniques, e.g. TEM elemental mappings at cross-sectioned samples) unprecedented insights into the microstructure evolution upon surface crystallization of model glasses will be gained.
Boratgläser zeigen im allgeneinen eine niedrige Glasübergangstemperatur, Tg, auf Grund der 3fach Koordination des Bors mit Sauerstoff. Der Zusatz von Alkali- und Erdalkalioxiden zur Glaszusammensetzung führt zunächst zu einem Anstieg von Tg auf Grund des Koordinationswechsels des Bors von der 3fach zur 4fach Koordination, was als Borsäureanomalie bekannt ist. Bei hohen Alkalioxidgehalten werden dann zunehmend Nichtbrückensauerstoffe (NBO) auf Kosten des tetraedrisch koordinierten Bor gebildet, was zur Senkung der Viskosität führt. Im Gegensatz zu diesem bekannten Verhalten der Alkalioxide (R2O) ist wenig über die Wirkung von Wasser in Boratgläsern bekannt. Daher sollen in dieser Arbeit die rheologischen Eigenschaf-ten von wasserreichen Kalk-Natron-Boratgläsern unter spezieller Berücksichtigung der Rolle des Wassers näher betrachtet werden. Zu diesem Zweck wurde Tg in Abhängigkeit des Na2O- und H2O-Gehalts mittels Differentialthermoanalyse (DTA) und Mikropenetrationsviskosimetrie untersucht. Die Ergebnisse zeigen, dass im Gegensatz zu Na2O Wasser in allen untersuchten Glä-sern und H2O-Konzentrationsbereichen (< 8 Gew.%) Tg deutlich verringert. Diese Beobachtung weist darauf hin, dass Wasser primär zur Bildung von NBO führt und kaum zu einem Koordina-tionswechsel des Bors beiträgt.